It is often desirable to seal the opening of a bottle, jar or other container opening using a sealing member or inner seal to maintain freshness and/or to indicate whether the container has been tampered with. Often a cap or other closure is then screwed or placed on the neck or other container opening. In use, a consumer typically removes the cap or other closure to gain access to the seal and removes or otherwise peels the seal from the container in order to dispense or gain access to its contents.
Initial attempts at sealing a container opening included an induction- or conduction-type inner seal covering the container's opening where the seal generally conformed to the shape of the opening such that a circular container opening was sealed with a round disk approximately the same size as the opening. These prior seals commonly had a lower heat activated sealing layer to secure a periphery of the seal to a rim or upper surface surrounding a container's opening. Upon exposing the seal to heat, the lower layer bonds to the container rim. In many cases, these seals included a foil layer to provide induction heat to activate the lower heat seal layer. These prior seals tended to provide good sealing, but can be difficult for a consumer to remove because there was nothing for the consumer to grab in order to remove the seal. Often, the consumer needed to pick at the seal's edge with a fingernail because there was little or no seal material to grasp.
Other types of seals for containers include a side tab or other flange that extends outwardly from a peripheral edge of the seal. These side tabs are generally not secured to the container rim and provide a grasping surface for a consumer to hold and peel off the seal. These side tabs, however, extend over the side of the container rim and often protrude into a threaded portion of the closure. If the side tab is too large, this configuration may negatively affect the ability of the seal to form a good seal. The side tabs (and often the seal itself) can be deformed or wrinkled when the closure or other cap is placed on the container due to contact between the closure and tabbed part of the seal. To minimize these concerns, the side tabs are often very small; thus, providing little surface area or material for a consumer to grasp in order to remove the seal.
Yet other types of seals include a sealing member having a tab defined on the top of the seal. One approach of these prior seals includes a partial layer of coated pressure sensitive adhesive to secure the tab to a layer of metal foil. This type of top-tabbed seal offers the advantage of a larger tab, which provides more grasping area for the consumer to hold and peel off the seal. These seals, however, have a non-uniform cross-section between the tabbed and non-tabbed sides of the seal because of the part layer of pressure sensitive adhesive. This non-uniform cross-section tends to cause problems when heat sealing to the container via an induction or conduction process. If sufficient heat is applied to activate the lower heat activated sealing layer on the tabbed side of the seal, then due to the additional layers on the non-tabbed side of the seal (i.e., the additional adhesive layer), the non-tabbed side often does not receive enough heat to adequately bond to the container. Conversely, if additional heating is applied in order to insure that both sides of the seal receive sufficient heat for bonding to the container rim, then the tabbed side of the seal often receives too much heat resulting in too strong of a bond to the container. If a portion of the lower heat activated sealing layer is bonded to the container too strongly, this heat-seal/container bond can exceed the bond strength of the adhesive holding the tab to the lower seal layers or even the tear strength of the tab material itself. This undesirable situation can result in the tab tearing or the tab separating from the lower seal layers upon a consumer using the tab to remove the seal. In other approaches, the seal may include a part paper or polymer layer forming the tab. This part layer may also form a non-uniform seal between the tabbed and non-tabbed sides. The part layer tends to provide extra insulation, which also leads to non-uniform heat seal bonding between the tabbed and non-tabbed sides of the seal.
Some top-tabbed-type seals may further include a foam layer in the lower seal portion positioned on top of any foil layer and under the tab to try and retain uniform heat between the tabbed and non-tabbed side of the seal even when the seal has a non-uniform cross-section. In these prior seals, the foam layer is advantageous in the lower seal laminate under the tab because it not only helps provide uniform heating to both sides of the lower heat seal layer, but with its close proximity to the foil, it further helps insulate and protect the upper layers and tab from damage or melting due to the heat experienced by the seal during heat sealing. In particular, the foam layer of these prior seals helps protect the adhesive layer that secures the tab to the lower seal laminate in these prior seals. Often, the adhesive layer used to secure the tab to the lower layers is also a heat activated adhesive layer rather than the pressure sensitive adhesive mentioned above. The heat activated adhesive layer has a lower melt point that renders it susceptible to melting during field use when an end user applies induction or conduction heat to secure the seal to a container. If the heat activated adhesive layer melts during container sealing, it can ooze or flow out of the seal and cause a free end of the tab to be bonded to the lower seal laminate. This is called tab-grab and is undesirable. End users, in some cases, will often overheat the seal to make sure that a good heat seal is formed. The foam layer under the heat-activated adhesive layer helps protect this heat-activated adhesive layer from possible overheating during container sealing.
However, while the foam layer may be advantageous in providing better seal performance, the foam layer is a weak point in the seal structure. Due to the location of the foam layer in the lower laminate above the foil and under the tab, a stress or failure point in the prior seal is created in the foam layer upon removal via the tab. In these cases, the foam layer typically needs to withstand the removal forces experienced on the lower portions of the seal upon tab pulling. During seal removal with the tab, these forces are caused by the tab pulling upwardly on the foam layer and because the tab is partially bonded to the foam. This stress is often exaggerated when a seal is aggressively applied to a container rim due to over sealing or in some circumstances where a stronger bond to the container may be needed. In such case, the consumer is then required to apply even a greater removal force to the tab. These prior seals with foam layers have a stress or failure point where the foam layer can split internally or the bond between the foam and upper seal layers can rupture resulting in tab and/or seal failure in some cases. In many cases, and especially when the seal is over sealed or aggressively sealed to a container, the foam layer under the tab and above the foil becomes a weaker link in the overall seal structure and a stress point prone to possible failure when exposed to removal forces.